The present invention relates to a processing method for the planetary erosion of an existing depression, whereby for a finishing or dressing operation, relatively cyclical lateral translational movement is carried out between a tool electrode and a workpiece, and whereby, using a final target value path, the actual movement is effected by regulating the width of the erosion gap via the deflection or travel radius.
Erosion methods are removal processing methods with which, by electrical discharge procedures between a tool electrode and an electrically conductive workpiece, under an operating medium, removal or erosion is brought about for the purpose of processing or machining. With a method of the aforementioned general type, the removal or erosion is effected by successive, chronologically separate, non-stationary or quasi-stationary electrical discharges.
During planetary erosion, three-dimensional translational movements between the tool electrode and the workpiece are superimposed over the linear movement that is conventional with immersion erosion (see "Industry-Anzeiger", Special Edition April 1981, page 109). The three-dimensional translational movement during planetary erosion effects improved operating conditions and results. Particularly advantageous is the possibility of being able to rough and finish with one and the same tool electrode. The finishing or dressing is effected with less discharge energy than is the case for roughing, so that the width of the gap between the tool electrode and the workpiece must be reduced. With immersion erosion, this is possible only by replacing the tool electrode, and with planetary erosion can be achieved via the translational movement. In addition, due to the additional translational movement, the flushing conditions in the operating medium are improved, so that disruptive defective discharges due to removed material between the tool electrode and the workpiece can be avoided. Furthermore, the width of the gap between the tool electrode and the workpiece can be rapidly controlled to avoid faulty discharges, because the electrode can also be withdrawn laterally. In addition, the lateral translational movement has a favorable effect upon the wear distribution of the tool electrode (see "Industry-Anzeiger", op. cit., page 109 et seq).
However, the movements during planetary erosion also cause special problems, which occur in particular with tool electrodes that have corners and edges. Above all, the so-called deflection or travel strategy must be taken into account, pursuant to which the movements between the tool electrode and the workpiece are carried out until the prescribed maximum geometry is achieved.
With the heretofore known methods, predominantly circular translational movements are carried out. Included herewith is the so-called stripping and the so-called expansion in stages, whereby after a radial translational movement or a radial translational step, circular translational movements are carried out. Also included in the above is the so-called continuous expansion, with which a helical translational movement is carried out (see "Blech Rohre Profile" 27 (1980) 9, pages 539 to 544).
During the expansion in stages, due to the increase of the radius that is constant for each planetary revolution, and due to the subsequent translational movement along an exactly circular path, a defined workpiece volume is removed or eroded during each planetary revolution. Superimposed over the planetary movement is a regulation of the width of the erosion gap, so that essentially the normal discharges that occur with average gap width take place, and neither the no-load pulses that are caused by large gap widths, nor the defective discharges that are caused by small gap widths, are brought about. With this travel strategy the regulation is essentially effected via the velocity or rate of the translational movement. In addition, under certain geometries of the tool electrodes greatly fluctuating size differences of the engagement surfaces can result, which have a considerable impact upon the performance of the process. For example, where the electrode has a cross-sectional configuration with corners, a continuous change occurs between surface and linear engagement, with the workpiece volume being eroded much slower at the surfaces than in the corner regions.
In contrast, during the continuous expansion the tool electrode is swung outwardly as far as possible relative to the workpiece in conformity with the respective width of the erosion gap, and at the same time is moved past the surface that is to be eroded at a nearly constant angular velocity. The regulation of the width of the erosion gap is under these conditions effected via the changing of the actual deflection or travel radius. A movement in the main immersion direction can also be linked herewith, as a result of which each volume element of the tool electrode moves along a cone-shaped shell surface relative to the workpiece.
The actual movement of the tool electrode relative to the workpiece in a plane perpendicular to the main immersion direction approaches the cross-sectional configuration of the tool electrode during the course of the processing due to the volume characteristics that are to be removed at the corners and surfaces of the depression. In so doing, toward the end of the processing, and especially in the corners, great no-load regions occur in which the tool electrode has achieved the maximum extent of expansion. Methods are known that, in order to avoid or reduce efficiency losses, control the velocity of the translational movement as a function of the processing state, whereby the no-load regions are swept over at a greater velocity (see "Industry-Anzeiger", op. cit., pages 112 et seq). However, during the course of the processing, there results with this heretofore known method great differences between the geometry of the final target value path and the geometrically distorted actual movement, whereby these differences can fluctuate greatly over a single planetary revolution as a function of the geometry of the tool electrode. This can overcharge the gap width regulating system, so that the number of defective discharges increase, which results in increased wear of the tool electrode. This can make it necessary to adjust the gap width regulating system, which adjustment occurs at the critical process regions and results in efficiency losses at the less critical process regions.
It is therefore an object of the present invention to provide an improved processing method for a continuous expansion with planetary erosion, with this method in particular making possible a favorable process performance and a shorter processing time.